Power generation system and method with partially recuperated flow path

ABSTRACT

The present disclosure relates to a power generation system and related methods that use supercritical fluids, whereby a portion of the supercritical fluid is recuperated.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation application of U.S. patentapplication Ser. No. 14/632,672, filed Feb. 26, 2015, now allowed,entitled Power Generation System And Method With Partially RecuperatedFlow Path, that claims priority to and the benefit of U.S. ProvisionalApplication No. 61/966,574, filed Feb. 26, 2014, the entire contents ofwhich are incorporated by reference into this application in theirentirety.

TECHNICAL FIELD

The present disclosure relates to a power generation system and relatedmethods that use supercritical fluids, and in particular, to a powergeneration system and related methods where a portion of thesupercritical fluid is recuperated.

BACKGROUND

Traditionally, thermodynamic power generation cycles, such as theBrayton cycle, employ an ideal gas, such as atmospheric air. Such cyclesare typically open in the sense that after the air flows through thecomponents of the cycle, it is exhausted back to atmosphere at arelatively high temperature so that a considerable amount heat generatedby the combustion of fuel is lost from the cycle. A common approach tocapturing and utilizing waste heat in a Brayton cycle is to use arecuperator to extract heat from the turbine exhaust gas and transferit, via a heat exchanger, to the air discharging from the compressor.Since such heat transfer raises the temperature of the air entering thecombustor, less fuel is required to achieve the desired turbine inlettemperature. The result is improved thermal efficiencies for the overallthermodynamic cycle. However, even in such recuperated cycles, thethermal efficiency is limited by the fact that the turbine exhaust gastemperature can never be cooled below that of the compressor dischargeair, since heat can only flow from a high temperature source to a lowtemperature sink. More recently, interest has arisen concerning the useof supercritical fluids, such as supercritical carbon dioxide (SCO2), inclosed thermodynamic power generation cycles. One such prior art system1 is illustrated in FIG. 1.

As shown in FIG. 1, the prior art power generation system 1 includescompressors, turbines, combustors and heat exchangers arranged in afirst Brayton cycle 402, in which the working fluid is a supercriticalfluid, and a second Brayton cycle 404, in which the working fluid isambient air. The system 1 therefore includes an SCO2 cycle flow path 406and air breathing cycle flow path 423, which may be separate from eachother.

In FIG. 1, the flow of SCO2 along flow path 406 is as follows.Initially, a stream A of supercritical fluid is supplied to the inlet ofa compressor 408. The supercritical fluid enters the inlet of thecompressor 408 after it has been cooled and expanded to a temperatureand pressure that is close to its critical point. The supercriticalfluid is supplemented by a supercritical fluid source 431. Aftercompression in the compressor 408, the stream B of SCO2 is heated in across cycle heat exchanger 410, which is connected to the SCO2 flow path406 and air breathing flow path 423. The stream C of heated SCO2 fromthe heat exchanger 410 is then directed to the inlet of a turbine 412,where the SCO2 is expanded and produces shaft power that drives both theSCO2 compressor 408 and an output device 416 by shaft 417. The outputdevice 416 can be a turboprop, turbofan, gearbox or generator. Afterexpansion in the turbine 412, the stream D of SCO2 is cooled in a secondcross cycle heat exchanger 418, also connected to the SCO2 flow path 406and air breathing flow path 423. The stream A of cooled SCO2 is returnedto the inlet of the compressor 408 via the flow path 406. In the airbreathing Brayton cycle 404, initially, ambient air 411 is supplied to acompressor 420. The stream E of compressed air from the compressor 420is then heated in the heat exchanger 418 by the transfer of heat fromthe SCO2 after the SCO2 has been expanded in the turbine 412. The streamF of heated compressed air is then directed to a combustor 424. Thecombustor 424 receives a stream 427 of fuel, such as jet fuel, dieselfuel, natural gas, or bio-fuel, is introduced by a fuel controller 428and combusted in the air so as to produce hot combustion gas. The streamG of the combustion gas from the combustor 424 is directed to the heatexchanger 410 where heat is transferred to the SCO2, as discussed above.After exiting the heat exchanger 410, the stream H of combustion gas isexpanded in a turbine 426, which produces power to drive the aircompressor 420, via shaft 421. After expansion in the turbine 426, thecombustion gas I is exhausted to atmosphere.

While the supercritical-ambient fluid cycle power generation system 1shown in FIG. 1 can be advantageous, the heat exchangers required totransfer heat between the supercritical fluid cycle and the ambientcycle may be large, expensive, and impractical to implement. Moreeffectively managing flow cycles can improve heat transfer efficiency inpower generation systems that employ supercritical fluid cycles.

SUMMARY

An aspect of the present disclosure is a method for generating power ina system that includes a supercritical fluid cycle having asupercritical fluid flowing therethrough, an air-breathing cycle havingair flowing therethrough that does not mix with the flow of thesupercritical fluid. The method includes the step of directing air alongthe air-breathing cycle to flow through a plurality of heat exchangers.The method includes compressing the supercritical fluid in asupercritical fluid compressor along the supercritical fluid cycle andsplitting the supercritical fluid discharged from the supercriticalfluid compressor into first and second discharge streams of compressedsupercritical fluid, such that the first discharge stream of compressedsupercritical fluid flows through a recuperating heat exchanger. Themethod includes mixing the supercritical fluid discharged from therecuperating heat exchanger with the second discharge stream ofcompressed supercritical fluid and directing a mixture of compressedsupercritical fluid through one of the plurality of heat exchangersarranged and into an inlet of a supercritical fluid turbine, such thatheat from the air along the air-breathing cycle is transferred to themixture of compressed supercritical fluid. The method includes splittingthe supercritical fluid discharged from the supercritical fluid turbineinto a first and second discharge streams of expanded supercriticalfluid such that the first discharge stream of expanded supercriticalfluid flows through the recuperating heat exchanger so as to heat thefirst discharge stream of compressed supercritical fluid. In addition,the method includes mixing the expanded supercritical fluid dischargedfrom the recuperating heat exchanger with the second discharge stream ofexpanded supercritical fluid. The mixture of expanded supercriticalfluid is directed toward the inlet of the supercritical compressor,wherein heat from the mixture of expanded supercritical fluid istransferred to the air of the air-breathing cycle, thereby cooling themixture of expanded supercritical fluid to approximately its criticalpoint.

Another aspect of the present disclosure is a system configured togenerate power. The system includes a supercritical fluid cycle. Thesupercritical fluid cycle includes a supercritical fluid compressorconfigured to receive and compress a supercritical fluid, asupercritical fluid turbine configured to receive and expand thesupercritical fluid, and a recuperating heat exchanger configured toreceive discharge streams from the supercritical fluid compressor andthe supercritical fluid turbine. The system also includes an airbreathing cycle configured to heat air flowing along the air breathingcycle. The system further includes a plurality of heat exchangersarranged so that supercritical fluid from the supercritical fluid cycleand air from the air breathing cycle passes therethrough but does notintermix. The system is configured to: 1) split the supercritical fluiddischarged from the supercritical fluid compressor into first and seconddischarge streams of compressed supercritical fluid, such that a) thefirst discharge stream of compressed supercritical fluid flows throughthe recuperating heat exchanger, and b) the second discharge stream ofcompressed supercritical fluid flows through one set of the plurality ofheat exchangers; and 2) split the supercritical fluid discharged fromthe supercritical fluid turbine into a first and second dischargestreams of expanded supercritical fluid such that a) the first dischargestream of expanded supercritical fluid flows through the recuperatingheat exchanger, and b) the second discharge stream of expandedsupercritical fluid flows through a different set of the plurality ofheat exchangers. Heat from the first discharge stream of expandedsupercritical fluid is transferred to the first discharge stream of thecompressed supercritical fluid in the recuperating heat exchanger.

Another aspect of the present disclosure is a system configured togenerate power. The system includes a supercritical fluid cycle. Thesupercritical fluid cycle includes a supercritical fluid compressorconfigured to receive and compress a supercritical fluid, asupercritical fluid turbine configured to receive and expand thesupercritical fluid, and a recuperating heat exchanger configured toreceive discharge streams from the supercritical fluid compressor andthe supercritical fluid turbine. The system also includes an airbreathing cycle configured to heat air flowing along the air breathingcycle. The system also includes a plurality of heat exchangers arrangedso that supercritical fluid from the supercritical fluid cycle and airfrom the an air breathing cycle passes therethrough but does notintermix, wherein a first heat exchanger of the plurality of heatexchangers is arranged to feed into an inlet of the supercritical fluidturbine, and a second heat exchanger of the plurality of heat exchangersis arranged to feed into an inlet of the supercritical fluid compressor.The first heat exchanger has a first heat capacity rate and the secondheat exchanger has a second heat capacity rate that is substantiallydifferent than the first heat capacity rate. Further, the system isconfigured to: 1) split the supercritical fluid discharged from thesupercritical fluid compressor into first and second discharge streamsof compressed supercritical fluid, such that a) the first dischargestream of compressed supercritical fluid flows through the recuperatingheat exchanger, and b) the second discharge stream of compressedsupercritical fluid flows through the first heat exchanger of theplurality of heat exchangers; and 2) split the supercritical fluiddischarged from the supercritical fluid turbine into a first and seconddischarge streams of expanded supercritical fluid such that a) the firstdischarge stream of expanded supercritical fluid flows through therecuperating heat exchanger, and b) the second discharge stream ofexpanded supercritical fluid flows through the second heat exchanger ofthe plurality of heat exchangers.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofan aspect, are better understood when read in conjunction with theappended diagrammatic drawings. For the purpose of illustrating theinvention, the drawings show an aspect that is presently preferred. Theinvention is not limited, however, to the specific instrumentalitiesdisclosed in the drawings. In the drawings:

FIG. 1 is a schematic diagram of a prior art power generation systemincorporating a supercritical fluid;

FIG. 2 is a schematic diagram of a power generation system according toan aspect of the disclosure;

FIG. 3 is a schematic diagram of a power generation system according toanother aspect of the disclosure;

FIG. 4 is a schematic diagram of a power generation system according toanother aspect of the disclosure;

FIG. 5 is a chart illustrating heat exchanger capacity rate ratios forsupercritical fluid and air according to the prior art power generationsystem illustrated in FIG. 1;

FIG. 6 is a chart showing delta temperature between supercritical flowand air flow along a heat exchanger from inlet-to-exit as a function offin location according to the prior art system illustrated in FIG. 1;

FIG. 7 is a chart illustrating temperature as a function of fin stationin a first heat exchanger according to an aspect of the disclosure; and

FIG. 8 is a chart illustrating temperature as a function of fin stationin a third heat exchanger along a partially recuperated cycle accordingto an aspect of the disclosure.

DETAILED DESCRIPTION

FIG. 2 is a schematic diagram of a power generation system 100 accordingto an aspect of the disclosure. The power generation system 100 includesa first closed Brayton cycle 102, in which the working fluid may be asupercritical fluid, and a second open Brayton cycle 104, in which theworking fluid may be ambient air. The first Brayton cycle 102 and thesecond Brayton cycle 104 include a supercritical fluid flow path 106 andan air fluid flow path 108, respectively. The flow paths 106 and 108are, in one aspect, separate so that little or no mixing occurs betweenthe supercritical fluid and air between the two flow paths 106 and 108.

The power generation system 100 includes compressors, turbines, one ormore combustors, and a plurality of heat exchangers connected along theflow paths 106 and 108. The heat exchangers include a plurality ofcross-cycle heat exchangers 132, 134, 136, and 138. As used herein, theterm “cross cycle heat exchanger” refers to a heat exchanger thatreceives air or both air and combustion gas from the air breathing cycle104 as well as a supercritical fluid from the supercritical fluid cycle102 and transfers heat between the fluids in the two cycles.Furthermore, the power generation system 100 includes a recuperatingheat exchanger 130 along the supercritical fluid flow path 106. As usedherein, the term “recuperating heat exchanger” refers to heat transfersbetween the supercritical fluid discharged from the SCO2 turbine and thesupercritical fluid discharged from the SCO2 compressor in thesupercritical fluid cycle 102. The power generation system 100 also mayinclude valves 122, flow meters 140, mixing junctions 124, and one ormore controllers configured to control operation of the system 100.

Initially, a stream 2 of supercritical fluid is supplied to the inlet ofa compressor 110, which may be an axial, radial, reciprocating or thelike type compressor. The compressor 110 may be referred to as firstSCO2 compressor 110. The compressor 110 includes a shaft 112 operablyconnected to a turbine 114. The turbine 114 may be referred to as firstSCO2 turbine 114. The flow meter 140 along the stream 2 measures a flowrate of the supercritical fluid supplied to the compressor inlet. Theflow meter 140 facilities control of total SCO2 mass in thesupercritical fluid cycle 102 as well as transient flow behavior. In oneaspect, the supercritical fluid enters the inlet of the SCO2 compressor110 after it has been cooled and expanded, as discussed below, to atemperature and pressure that is close to its critical point. The term“supercritical fluid” refers to a fluid in which distinct liquid andgaseous phases do not exist, and term “critical point” of asupercritical fluid refers to the lowest temperature and pressure atwhich the substance can be said to be in a supercritical state. Theterms “critical temperature” and “critical pressure” refer to thetemperature and pressure at the critical point. For carbon dioxide, thecritical point is approximately 304.2° K and 7.35 MPa. In one aspect,the supercritical fluid entering the compressor 110 is cooled to withinat least ±2° K of its critical point. In a further aspect, thesupercritical fluid entering the compressor 110 is cooled to within ±1°K of its critical point. In yet another aspect, the supercritical fluidentering the compressor 110 is cooled to within ±0.2° K of its criticalpoint.

After compression in the SCO2 compressor 110, the discharge stream 4 ofthe supercritical fluid is split into first and second portions as firstand second discharge streams 6 and 8. The streams 6 and 8 may bereferred to herein as compressor discharge streams 6 and 8. The splitpermits the first portion of the discharge stream 4 from the compressor110 to be recuperated and the remaining portion to be heated directly bya series of heat exchangers 134 and 132 by air fluid cycling through theflow path 108. As illustrated, the discharge stream 4 is split via valve122 a which can be in electronic communication with a controller (notshown). The controller operates or actuates the valve 122 a to directflow through the flow path 106 as needed. In one aspect, the valve 122 ais configured to direct between 55% to about 75% of the discharge stream4 into the first discharge stream 6. The balance of the flow of thedischarge stream 4 is directed to the second discharge stream 8. Inanother aspect, the valve 122 a is configured to direct about 67% of thedischarge stream 4 into the first discharge stream 6.

The first discharge stream 6 of the supercritical fluid is directed tothe recuperating heat exchanger 130 where heat is transferred from theheated SCO2 exiting turbine 116 to the first discharge stream 6. Thestream 19 of the heated SCO2 discharged from the recuperating heatexchanger 130 is directed to the junction 124 a and mixed with thestream 10 of heated SCO2 that exits the cross-cycle heat exchanger 134.

The second discharge stream 8 from the SCO2 compressor 110 is directedto the cross cycle heat exchanger 134. In the cross cycle heat exchanger134, the heat from the combustion gas in flow path 108 is transferred tothe second discharge stream 8 of SCO2. The stream 10 discharged fromheat exchanger 134 mixes with stream 19 of SCO2 from recuperating heatexchanger 130 at junction 124 a, as discussed above. The junction 124 amay be joint that is connected to conduits or it may include a mixingapparatus.

The mixed stream 12 is supplied to the cross cycle heat exchanger 132.In the cross cycle heat exchanger 132, heat is transferred from thecombustion gas in the flow path 108 to the mixed stream of SCO2. Thecross cycle heat exchanger 132 discharges the stream 14 of heated SCO2.

The stream 14 of heated SCO2 from the heat exchanger 132 is directed tothe inlet of the first SCO2 turbine 114. The first SCO2 turbine 114 maybe an axial, radial, mixed flow, or the like type turbine. The firstSCO2 turbine 114 expands the SCO2 and produces shaft power that drivesthe SCO2 compressor 110, via shaft 112. After expansion in the firstSCO2 turbine 114, the stream 15 is cycled through a second SCO2 turbine116 that produces shaft power for a generator 120, via the shaft 118.The generator 120 can provide output power for the system 100. In analternate aspect, the cycle 102 may include one turbine 114 with theshaft 118 connected to the turbine 114 and the generator 120. In such anaspect, the discharge stream 16 would discharge from the turbine 114into a valve 122 b.

The discharge stream 16 from the second SCO2 turbine 116 may be splitinto second and first portions as the discharge stream 18 and thedischarge stream 22. The discharge stream 18 and the discharge stream 22may be referred to as second and first discharge streams 18 and 22. Asillustrated, the valve 122 b can spilt the discharge stream 16 into thesecond and first discharge streams 18 and 22. The controller operates oractuates the valve 122 b. In one aspect, the valve 122 b is configuredto direct between 70% to about 90% of the discharge stream 16 into thefirst discharge stream 22. The balance of the flow of the dischargestream 16 is directed to the second discharge stream 18. In anotheraspect, the valve 122 b is configured to direct about 80% of thedischarge stream 16 into the first discharge stream 22. Regardless ofhow the SCO2 turbine discharge stream 16 is spilt, the second dischargestream 18 is directed to the cross cycle heat exchanger 136 and cooledby the flow of air passing through the heat exchanger 136 along the flowpath 108.

The first discharge stream 22 is directed to the recuperating heatexchanger 130, where heat from the discharge stream 22 is transferred tofirst discharged stream 6 from the SCO2 compressor 110. In other words,the recuperating heat exchanger 130 cools the discharge stream 22 ofSCO2. The discharge stream 24 of the cooled SCO2 from the recuperatingheat exchanger 130 is mixed with an incoming stream 20 from the heatexchanger 136 at a junction 124 b. From the junction 124 b, the mixedstream 26 is directed to the cross-cycle heat exchanger 138 which may beoptional). For instance, mixed stream 26 may be directed directly to thecompressor 110. As noted above, in the cross-cycle heat exchanger 138,heat from the mixed stream 26 of SCO2 is transferred to the flow path108 of the air cycle 104. The stream 28 of cooled SCO2 is directedthrough a cooler 126 (which may be optional) and is returned to theinlet of the SCO2 compressor 110 as stream 2. Additional SCO2 from asupply 109 can be introduced into the stream 2 of SCO2 directed to theSCO2 compressor 110 to make up for any leakage of SCO2 from the system.In any event, the SCO2 stream 2 is returned to the inlet of thecompressor 110 and the steps of compressing-heating-expanding-coolingare repeated.

Continuing with FIG. 2, the air breathing cycle 104 portion of theoverall system 100 forms an open flow path 108. Initially, ambient air101 is supplied to an air breathing compressor 150 which may be anaxial, radial reciprocating, or like type compressor. The compressor 150includes a shaft 152 operably connected to a turbine 154. The stream 30of compressed air from the compressor 150 is then heated in the heatexchanger 138 (which may be optional) by the transfer of heat from themixed stream 26 of SCO2 discharged from the turbine 116 via the heatexchangers 130 and 136 as discussed above. The stream 32 of heatedcompressed air is then directed to the heat exchanger 136, where heatfrom the stream 18 of SCO2 (from SCO2 turbine 116) is transferred to thestream 32 of compressed air. The discharge stream 34 is the directed tothe combustor 158. The combustor 158 raises the temperature of thecompressed air stream 34 above the required temperature at the turbineinlet of turbine 154. The compressor 150 can operate via shaft 152powered by turbine 154. The combustor 158 can receive a stream of fuel103, such as fossil fuels or other fuel type. The combustor 158 canoperate by means of a solar collector or nuclear reactor to producesystem heat or some may other heat source of heat including combustionof waste, biomass, or bio-derived fuels. The discharge stream 36 of thecombustion gas from the combustor 158 may be directed to the turbine154, where it is expanded. The stream 40 of expanded hot combustion gasis directed to the heat exchanger 132, where heat is transferred fromthe hot combustion gas to the mixed stream 12 of SCO2 discussed above.After exiting the heat exchanger 132, the stream 41 of hot combustiongas is directed to the heat exchanger 134, where heat is transferredfrom the hot combustion gas to the discharge stream 8 of SCO2 from theSCO2 compressor 110, as discussed above. The discharge stream 107 of theheat exchanger 134 may be exhausted into atmosphere.

In operation, the power generation system 100 will be described withreference to predicted results. For instance, the heat capacity rate canbe determined by multiplying mass flow rate times the specific heat Cp,or mdot*Cp. The heat exchangers 136 and 134 have mis-matched heatcapacity rates because they operate in the regime of temperatures wheresupercritical fluid, such as SCO2, has a more linear and flat specificheat Cp curve. See for example FIG. 4. Because the heat capacity ratesat these locations are not well matched, an air mass flow rate in theair breathing cycle 104 can be lower compared to prior art system 1shown in FIG. 1. An aspect of the present disclosure includes storingheat by creating a large difference in temperature ranges of the twoflows and mis-matching the heat capacity rate, which can avoid the heatpinch point problem associated with the prior art system. In oneexample, the supercritical fluid cycle 102 in the power generationsystem 100 can have a mass flow rate between about 30 and 35 Kg/sec. Theair cycle 104 in the power generation system 100 can have a mass flowrate between about 7.5 and about 16.0 Kg/sec. However, the mass flowrates stated herein are not considered limiting. They may be higher orlower than the ranges provided. Furthermore, the power generation system100 is configured to have a ratio of air mass flow rate to supercriticalfluid mass flow rate of between about 0.25 and 0.50. In one aspect, theratio of the mass flow rates is approximately 0.30. Accordingly, themass flow rates for the air in the air-breathing cycle 104 are generallylower compared to typical power generation systems. In just one example,the air mass flow rates are about 75% below the air mass flow rates in aprior art power generation system 1, such as the aspect shown in FIG. 1and described above. Reduced air mass flow can result in a substantialreduction in heat exchanger size, footprint, cost, weight, parasiticpower requirements and the like.

Turning to FIG. 3, which is a schematic diagram of a power generationsystem 200 according to another aspect of disclosure configured togenerate power and heat. The power generation system 200 is similar tothe aspect shown in FIG. 2, and includes a first or supercritical fluidcycle 202 and a second or air-breathing cycle 204. The first and secondcycles 202 and 204 include a supercritical fluid flow path 206 and anair fluid flow path 208, respectively, that are in one aspect separatefrom each other such the supercritical fluid and air does not intermix.Furthermore, the power generation system 200 includes compressors,turbines, one or more combustors, at least one recuperating heatexchanger 230, a plurality of cross-cycle heat exchangers 232, 234, 236,and 238, as well as valves 222, flow meters, mixing junctions 224, andone or more a controllers configured to control operation of the system.

Initially, a stream 42 of supercritical fluid is supplied to the inletof a compressor 210. The compressor 210, sometimes referred to as thefirst SCO2 compressor 210, includes a shaft 212 operably connected tothe first turbine 214, also referred to as the first SCO2 turbine 214.An optional flow meter (not shown) can be used measure the flow rate ofthe fluid supplied to the compressor inlet. The stream 42 of thesupercritical fluid enters the inlet of the compressor 110 after it hasbeen cooled and expanded to a temperature and pressure that is close toits critical point.

After compression in the compressor 210, the stream 44 of supercriticalfluid is split into first and second portions as streams 46 and 48. Thestreams 46 and 48 may be referred to as first and second dischargestreams 46 and 48, respectively. A valve 222 a can split the stream 44into the first and second discharge streams 46 and 48. The firstdischarge stream 46 of the supercritical fluid is supplied to therecuperating heat exchanger 230. In the recuperating heat exchanger 230,heat is transferred from the heated SCO2 discharge from a turbine 216 tothe first discharge stream 46 from the SCO2 compressor 210. The stream50 of heated SCO2 discharged from heat exchanger 230 is directed to ajunction 224 a and mixed with the stream 74 of heated SCO2 from across-cycle heat exchanger 234.

The second discharge stream 48 is directed to a valve 222 b, whichdirects the stream 70 through an optional heat exchanger 233 and intothe cross cycle heat exchanger 234. Exchanger 233 can be used to capturewaste heat from avionics and weapons systems that are installed inmoving platforms like aircraft, surface vessels, etc. The system 200 maynot include heat exchanger 233 in every application or implementation.In the cross cycle heat exchanger 234, the heat is transferred from thecombustion gas in the flow path 208 to the discharge stream 70 of SCO2.The stream 74 discharged from heat exchanger 234 mixes with stream 50 ata junction 224 a. The junction 224 a may be a joint or may include amixing apparatus. The stream 51 is supplied to another junction 224 band combined with the discharge stream 72 from a cooler 219. The valve222 b also may direct a portion of a second discharge stream 48 to thecooler 219 disposed along a shaft 218. The discharge stream 72 from thecooler 219 is routed to the junction 224 b combined with the stream 51into mixed the stream 52. The mixed stream 52 is supplied to the crosscycle heat exchanger 232. In the cross cycle heat exchanger 232, heatfrom the combustion gas in the flow path 108 is transferred to the mixedstream 52. The discharge stream 54 of heated SCO2 from the cross cycleheat exchanger 232 is directed to the inlet of the first SCO2 turbine214.

The first SCO2 turbine 214 expands the SCO2 and produces shaft powerthat drives the SCO2 compressor 210, via shaft 212. After expansion inthe first SCO2 turbine 214, the stream 56 is cycled through a secondSCO2 turbine 216 that produces shaft power for a generator 220, via theshaft 218. The generator 220 can provide output power for the system200. Alternatively, the stream 56 can bypass the turbine 216. Asillustrated, a valve 222 c divides the stream 56 into a stream 57directed toward the turbine 216 and the stream 58 directed toward theheat exchangers 130 and 236. The stream 59 discharged from the turbine216 flows to a junction 224 c and is combined with the stream 58 todefine a discharge stream 60.

The discharge stream 60 is directed to a valve 222 d, which splits thedischarge stream 60 from the turbine 216 into a second discharge stream62 and a first discharge stream 66. The second discharge stream 62 isdirected to a cross cycle heat exchanger 236 and heated by the flow ofair along a flow path 208 through the heat exchanger 236. The dischargestream 64 discharged from the heat exchanger 236 is directed toward theheat exchanger 238. The first discharge stream 66 of SCO2 is directed tothe recuperating heat exchanger 230, where its heat is transferred tothe first discharge stream 4 of SCO2 from the SCO2 compressor 210. Thedischarge stream 68 from the recuperating heat exchanger 230 is mixedwith a discharge stream 64 from the heat exchanger 236 at a junction 224d, forming a mixed stream 69. The mixed stream 69 of SCO2 is directed tothe heat exchanger 238, where heat from the SCO2 fluid is transferred tocompressed air along the flow path 208 of the air cycle 204. The stream28 of cooled SCO2 is directed through a cooler 226 (which may beoptional) and is returned to the inlet of the SCO2 compressor 210 viathe flow path 206. A water input 225 a may supply water to a cooler 226.The output stream 225 b of the cooler 226 is heated water, which can beused as a heat source. Additional SCO2 from a supply 207 can beintroduced into the stream 42 of SCO2 directed to the compressor 210 tomake up for any leakage of SCO2 from the system. In any event, the SCO2stream 202 is returned to the inlet of the compressor 210 and the stepsof compressing-heating-expanding-cooling are repeated. In an alternativeaspect, yet another heat exchanger 239 a is placed along stream 68. Awater input 239 b may supply water to exchange 239 a. The output stream239 c of the heat exchanger 239 a is heated water which can be used as aheat source for district heating. District heating generally requireswater temps of 180 F or better, including heat exchanger 239 a can helpensure output stream temperature of about 180 F or better, as opposed tosystem 200 that include only the cooler 226. Accordingly, the system 200may include cooler 226 or heat exchanger 239 a. In still otheralternatives, the system 200 can include both cooler 226 and heatexchanger 239 a.

Continuing with FIG. 3, the air breathing cycle 104 portion of theoverall system 200 forms open flow path 208. Initially, ambient air 201is supplied to a forced draft fan 250 which may be axial, radial,reciprocating, or similar type compressor. The forced draft fan 250 isdriven by shaft 252 powered by a power source 254. The power source 254can be a motor. The stream 80 of compressed air from the forced draftfan 250 is then heated in the heat exchanger 238 by the transfer of heatfrom the mixed stream 69 of SCO2 (discharged from turbine 216 and cooledin the heat exchanger 230 and 236). The air stream 82 of heatedcompressed air is then directed the heat exchanger 236, where heat fromthe second discharge stream 62 of heated SCO2 is transferred to an airstream 82. The air stream 84 is fed to a combustor 258 into which a fuel203 (such as a fossil fuel, heat from solar conductor, nuclear reactor,or the like is supplied) is introduced by a fuel controller andcombusted in the air so as to produce hot combustion gas. The stream 86of the combustion gas from the combustor 258 is directed to a heatexchanger 232, where heat is transferred from the stream 86 of hotcombustion gas to the mixed stream 52 of SCO2 discussed above. Thestream 88 of hot combustion gas directed to the heat exchanger 234,where heat is transferred from the hot combustion gas to the stream 74of compressed SCO2, as discussed above. The discharge stream 90 of theheat exchanger 234 may be directed to an induced draft fan 260, whichmay be a compressor. The induced draft fan 260 may be connected to ashaft 262, which is powered by a power source 264, such as a motor. Thestream of gas may be exhausted from the induced draft fan 260 toatmosphere. The purpose of both forced draft fan 250 and induced draftfan 260 is to drive flow through the heat exchangers and combustor andto overcome the pressure drop associated with them. It should beappreciated that the forced draft fan 250 may not be needed based on thetype of fuel burnt in the combustor. For instance, a forced draft fan250 is useful when it is desirable for the combustion zone to besub-atmospheric pressure in the case of burning biomass where fuel isintroduced through an open door. If, however, the combustor can bepressurized, as in the case of burning fossil fuels, the induced fan 260is not necessary.

In operation and as described above with respect to the system 100, theheat exchangers 236 and 234 have mis-matched heat capacity rates becausethey both operate in the regime of temperatures where the supercriticalfluid has a more linear and flat heat capacity rate curve. Because theheat capacity rates at these locations are not well matched, an air massflow rate in the air breathing cycle 204 can be lower compared to priorart system 1 shown in FIG. 1. An aspect of the present disclosureincludes storing heat by creating a large difference in temperatureranges of the two flows and mis-matching the heat capacity rate, whichcan avoid the heat pinch point problem associated with the prior artsystem. In one example, the supercritical fluid cycle 202 in the powergeneration system 200 can have a mass flow rate between about 30 and 35Kg/sec. The air cycle 204 in the power generation system 200 can have amass flow rate between about 7.5 and about 16.0 Kg/sec. However, themass flow rates stated herein are not considered limiting. They may behigher or lower than the ranges provided. Furthermore, the powergeneration system 200 is configured to have a ratio of air mass flowrate to supercritical fluid mass flow rate of between about 0.25 and0.50. In one aspect, the ratio of the mass flow rates is approximately0.30. Accordingly, the mass flow rates for the air in the air-breathingcycle 204 are generally lower compared to typical power generationsystems. In just one example, the air mass flow rates are about 75%below the air mass flow rates in a prior art power generation system 1.

Turning to FIG. 4, which is a schematic diagram of a power generationsystem 300 according to another aspect of disclosure. The powergeneration system 400 is substantially similar to the power generationsystem 100 shown in FIG. 2 and described above. The description belowwill use the same reference numbers to identify elements that are commonbetween power generation system 100 and power generation system 300.Accordingly, the power generation system 300 a supercritical fluid cycle402 and an air breathing cycle 404. Furthermore, the power generationsystem 300 includes compressors, turbines, one or more combustors, and aplurality of heat exchangers connected along the flow paths 306 and 308.The heat exchangers include a plurality of cross-cycle heat exchangers132, 134 and 136 along the flow path 308, and a recuperating heatexchanger 130 along the supercritical fluid flow path 306. The powergeneration system 300 also may include valves 122, flow meters 140,mixing junctions 124, and one or more controllers configured to controloperation of the system 300. As noted above, the power generation system300 operates substantially similar to the power generation system 100.

In accordance with the alternative aspect of the disclosure, however,the power generation system 300 does not include a terminal heatexchanger 138 that discharges stream 28 toward the inlet of thecompressor 110 (see FIG. 2). In accordance with the power generationsystem 300, the valve 122 b divides the discharge stream 16 from thesecond SCO2 turbine 116 into second discharge stream 318 and a firstdischarge stream 322. In one aspect, the controller operates or actuatesthe valve 122 b to direct between 70% to about 90% of the dischargestream 16 into the first discharge stream 322. The balance of the flowof the discharge stream 16 is directed to the second discharge stream318. In another aspect, the valve 122 b is configured to direct about80% of the discharge stream 16 into the first discharge stream 322.Regardless of how the SCO2 turbine discharge stream 16 is spilt, thesecond discharge stream 318 is directed to the cross cycle heatexchanger 136 and cooled by the flow of air passing through the heatexchanger 136 along the flow path 408.

The first discharge stream 322 is directed to the recuperating heatexchanger 130, where heat from the discharge stream 322 is transferredto first discharged stream 6 from the SCO2 compressor 110. The dischargestream 324 of the cooled SCO2 from the recuperating heat exchanger 130is mixed with an incoming stream 20 from the heat exchanger 136 at ajunction 124 b. From the junction 124 b, the mixed stream 328 isdirected to the compressor 110. As illustrated, the stream 328 of cooledSCO2 is directed through a cooler 126 (which may be optional) and isreturned to the inlet of the SCO2 compressor 110 as stream 2. In anyevent, the SCO2 stream 2 is returned to the inlet of the compressor 110and the steps of compressing-heating-expanding-cooling are repeated.

Continuing with FIG. 4, the air breathing cycle 304 portion of theoverall system 300 forms an open flow path 408. Initially, ambient air101 is supplied to an air breathing compressor 150. The stream 30 ofcompressed air from the compressor 150 is directed to heat exchanger 136and is heated by the transfer of heat from the stream 318 of SCO2discharged from the turbine 116. The discharge stream 34 is the directedto the combustor 158. The discharge stream 36 of the combustion gas fromthe combustor 158 may be directed to the turbine 154, where it isexpanded. The stream 40 of expanded hot combustion gas is directed tothe heat exchanger 132, where heat is transferred from the hotcombustion gas to the mixed stream 12 of SCO2 as discussed above. Afterexiting the heat exchanger 132, the stream 41 of hot combustion gas isdirected to the heat exchanger 134, where heat is transferred from thehot combustion gas to the discharge stream 8 of SCO2 from the SCO2compressor 110. The discharge stream 107 of the heat exchanger 134 maybe exhausted into atmosphere.

The power generation system 300 requires fewer cross-cycle heatexchangers compared to other aspects of the present disclosure.Furthermore, it should be appreciated that the power generation system200 can be implemented without the need for heat exchanger 238. In suchan example, the stream 69 is directed to directly to the optional coolerand then the inlet of compressor 210. Furthermore, on the air-breathingcycle 204, the discharge stream 80 is directed into heat exchanger 236and the cycle continues as disrobed above.

The power generation systems 100, 200, and 300 described above haveseveral advantages over typical supercritical power generation systemsand/or or other non-supercritical fluid based systems. Reduced heatexchanger size, improved thermal efficiency, and lower thermal signatureat exhaust are a few notable improvements. The alternative heatexchanger flow strategy-whereby SCO2 discharge flows from the SCO2compressor and SCO2 turbine are spilt-mitigates a so-called heatexchanger “pinch point” in the prior art system 1. More specifically,the prior art system 1 has a variable heat capacity mismatch at heatexchanger 418 (FIG. 1) on the low pressure side. The variable mismatchis based on a mismatch between the heat capacity rate of the air andSCO2 flows that are exchanging heat in the heat exchanger 418. Forinstance, as shown in FIG. 4, the air in the heat exchanger 418 has afairly linear heat capacity rate curve across its operatingtemperatures. The supercritical fluid, however, has a spike in the heatcapacity rate at the lower temperature range where the SCO2 dischargeend of the heat exchanger 418 operates. The effect of this spike in heatcapacity rate is illustrated in FIG. 6. FIG. 6 displays deltatemperature (ΔT) between the inlet and exit ends of the heat exchanger418 for both SCO2 flow and air flow as function of S-fins from the SCO2inlet. The different curves are different sized heat exchangers. Forinstance, the curve “100-Sfin Hx” would indicate a larger heat exchangercompared to the heat exchanger associated with the “50-Sfin Hx” curve.As noted above, the spike heat capacity rate of SCO2 at lowertemperature at the SCO2 discharge end indicates that the heat exchangerlength should be increased in order to create effective heat transfer.But as shown in FIG. 6, a relative low ΔT observed from 50-fins locationto about the 100-fins location relative to the SCO2 inlet end of heatexchanger for the 100-Sfin HX curve. This suggests that the section ofthe heat exchanger which is doing the least amount of heat transfer isactually elongated. The result is that the prior art system 1 requiresfairly large heat exchangers with limited or low performance andsometimes high loss of pressure, which can be detrimental to the systemperformance. Furthermore, large approach temperatures at either end ofthe heat exchanger 418 illustrate that a significant amount of heat isleft un-transferred.

As described above, the power generation systems 100, 200, 300 splitsthe discharge flows of SCO2 from the SCO2 compressor 110, 210, and theSCO2 turbine 116, 216, between: A) the recuperating heat exchanger 130,230, and B) heat exchangers that feed into the respective inlets of theSCO2 turbine and SCO2 compressor. This split, in conjunction with thearrangement of the air-breathing cycle 104, 204, 304 results in airflowstream (see stream 40 in FIGS. 2 and 4 and stream 86 in FIG. 3) on theinlet side of the SCO2 turbines with a temperature above the desiredtemperature of SCO2 stream at inlet of SCO2 turbine 114, 214.Furthermore, the splitting of SCO2 turbine discharge flow and SCO2compressor discharge flow allows for the intentional mismatch of heatcapacity rate at heat exchanger 138, 238 and heat exchanger 132, 232, asin FIGS. 2 and 3. For the power generation system 300 shown in FIG. 4,this intentional mismatch of heat capacity rate would be between heatexchanger 136 and heat exchanger 132. This in turn, permits somewhatlarge approach temperatures at the hot end of the heat exchanger 132,232 and the cool end of heat exchanger 138, 238 in FIGS. 2 and 3 andcool end of heat exchanger 136 in FIG. 4. The large approachtemperatures alleviate the “pinch point” issue for these particular heatexchangers as used in prior art systems. For the power generationsystems 100 and 200 shown in FIGS. 2 and 3, the heat exchangers 134, 234and 136, 236, however, have fairly well matched heat capacity ratesbecause they operate in a range where SCO2 has a more linear Cp curve.In any event, the high approach temperatures at heat exchanger 132, 232and heat exchanger 138, 238 increase the amount of heat exchanged perunit area of heat exchanger, further reducing heat exchanger size. Andin at least some instances allows the elimination of the heat exchanger138, 238, as in the power generation system 300 shown in FIG. 4.

System heat can be added to by means of combustion of fossil fuels, asolar collector, a nuclear reactor, and/or similar heat source, therebyraising the temperature of the air flow to a value above the requiredhigh temperature at the inlet of the SCO2 turbines. Furthermore, becausethe heated combustion gas passes the majority of its heat to SCO2streams via the heat exchangers 134, 234 and 132, 232, very low exhaustgas temperatures result and thus reductions in thermal signature forapplications where this is important, e.g., such as militaryapplications. And because of the low compressibility factor associatedwith supercritical fluids, the discharge temperature from the SCO2compressor is comparatively low and therefore ideal for receiving theheat energy from the heated combustion gas at the heat exchanger 134,234 and the SCO2 discharge flow at the recuperating heat exchanger 130,230. These attributes result is high thermal efficiency of the system.

In alternative aspects, a power generation system includes more than onesupercritical fluid cycle. In one example, the power generation systemcan include first and second supercritical fluid cycles, whereby one orboth of the first and second supercritical fluid cycles spilt the SCO2discharge from the SCO2 turbine and SCO2 compressor between A) arecuperating heat exchanger like 130, 230, and B) respective heatexchanger in-line with inlets of a SCO2 turbine and a SCO2 compressor.In still other alternative aspects, a power generation system includesone or more air breathing cycles. In still other aspects, the airbreathing cycle can include one or more reheat cycles. In still otheraspects, a power generation system includes a vacuum cycle with one ormore SCO2 cycles. In still other aspects, a power generation systemincludes steam injection. In still other aspects a power generationsystem includes a bottoming cycle using the low pressure dischargestream of heat exchangers 130, 230 as a heat source.

Furthermore, the power generation system 100, 200, 300 an includevarious SCO2 and air breathing cycles as disclosed in U.S. Patent App.Pub. No. 2013/0180259 (the 259 publication) in combination with thealternative flow strategy as described herein. The disclosure of theSCO2 and the air breathing cycles in the 259 publication that are notinconsistent with the flow strategies as described above areincorporated herein by reference in its entirety.

In another alternative aspect, the power generation system 100, 200 asdescribed herein includes a SCO2 turbine assembly that includes an eddycurrent torque coupling as disclosed in the 259 publication. Thedisclosure of the eddy current torque coupling in the 259 publication isincorporated by reference into this application in its entirety.

Applications for the power generation systems 100, 200, 300 include butare not limited to aircraft engines (such as turbo-fan, turbo-prop, orturbo-shaft engines), ground based electric power generators, navalpropulsion systems, ground transportation engines, etc. Furthermore,other applications can include power and heat generation, such as steamand hot water. The systems can be used for any other application whereshaft power is required.

The foregoing description is provided for the purpose of explanation andis not to be construed as limiting the invention. While the inventionhas been described with reference to preferred aspects or preferredmethods, it is understood that the words which have been used herein arewords of description and illustration, rather than words of limitation.Furthermore, although the invention has been described herein withreference to particular structure, methods, and aspects, the inventionis not intended to be limited to the particulars disclosed herein, asthe invention extends to all structures, methods and uses that arewithin the scope of the appended claims. Those skilled in the relevantart, having the benefit of the teachings of this specification, mayeffect numerous modifications to the invention as described herein, andchanges may be made without departing from the scope and spirit of theinvention as defined by the appended claims.

The invention claimed is:
 1. A system configured to generate power,comprising: an air breathing cycle configured to heat air flowing alongthe air breathing cycle; a supercritical fluid cycle including asupercritical fluid compressor configured to receive and compress asupercritical fluid, a supercritical fluid turbine configured to receiveand expand the supercritical fluid, at least one recuperating heatexchanger, a first valve configured to direct at least a portion of thecompressed supercritical fluid discharged from the supercritical fluidcompressor to the at least one recuperating heat exchanger, and a secondvalve configured to direct at least a portion of the expandedsupercritical fluid discharged from the supercritical fluid turbine tothe at least one recuperating heat exchanger; one or more controllers inelectronic communication with the first valve and the second valve, theone or more controllers configured to operate the first valve and thesecond valve so as to direct the flow of the supercritical fluid throughthe supercritical fluid cycle; and a plurality of cross cycle heatexchangers arranged so that supercritical fluid from the supercriticalfluid cycle and air from the air breathing cycle passes therethrough butdoes not intermix.
 2. The system of claim 1, wherein the first valve isfurther configured to split the compressed supercritical fluiddischarged from the supercritical fluid compressor into first and seconddischarge streams of compressed supercritical fluid, such that a) thefirst discharge stream of compressed supercritical fluid is directed tothe at least one recuperating heat exchanger, and b) the seconddischarge stream of compressed supercritical fluid is directed to atleast one cross cycle heat exchanger, and the second valve is furtherconfigured to direct the expanded supercritical fluid discharged fromthe supercritical fluid turbine into first and second discharge streamsof expanded supercritical fluid, such that a) the first discharge streamof expanded supercritical fluid is directed to the at least onerecuperating heat exchanger, and b) the second discharge stream ofexpanded supercritical fluid is directed to at least one different crosscycle heat exchanger.
 3. The system of claim 2, wherein the one or morecontrollers operates the first valve to direct between 55% to 75% of thecompressed supercritical fluid discharged from the supercritical fluidcompressor into the first discharge stream of compressed supercriticalfluid.
 4. The system of claim 2, wherein the one or more controllersoperates the first valve to direct about 67% of the compressedsupercritical fluid discharged from the supercritical fluid compressorinto the first discharge stream of compressed supercritical fluid. 5.The system of claim 2, wherein the one or more controllers operates thesecond valve to direct between 70% to 90% of the expanded supercriticalfluid discharged from the supercritical fluid turbine into the firstdischarge stream of expanded supercritical fluid.
 6. The system of claim2, wherein the one or more controllers operates the second valve todirect about 80% of the expanded supercritical fluid discharged from thesupercritical fluid turbine into the first discharge stream of expandedsupercritical fluid.
 7. The system of claim 1, further comprising: aneddy current torque coupling; and a third valve, the third valveconfigured to direct at least a portion of the compressed supercriticalfluid discharged from the supercritical fluid compressor to the eddycurrent torque coupling, wherein the one or more controllers is anelectronic communication with the third valve, and the one or morecontrollers is configured to operate the third valve so as to direct theflow of the supercritical fluid through the supercritical fluid cycle.8. The system of claim 2, further comprising: an eddy current torquecoupling; and a third valve, the third valve configured to split thesecond discharge stream of compressed supercritical fluid into third andfourth discharge streams of compressed supercritical fluid, such that a)the third discharge stream of compressed supercritical fluid is directedto the eddy current torque coupling, and b) the fourth discharge streamof compressed supercritical fluid is directed to the at least one crosscycle heat exchanger, wherein the one or more controllers is anelectronic communication with the third valve, and the one or morecontrollers is configured to operate the third valve so as to direct atleast a portion of the second discharge stream of compressedsupercritical fluid into the third discharge stream of compressedsupercritical fluid and to direct the balance of the second dischargestream of compressed supercritical fluid into the fourth dischargestream of compressed supercritical fluid.
 9. The system of claim 8,further comprising: an eddy current torque coupling discharge stream;and first and second temperature sensors, the first and secondtemperature sensors in electronic communication with the one or morecontrollers, wherein the first temperature sensor is configured tomeasure a temperature of the fourth discharge stream of compressedsupercritical fluid, and the second temperature sensor is configured tomeasure a temperature of the eddy current torque coupling dischargestream, wherein the one or more controllers are further configured tooperate the third valve in response to the temperature of the fourthdischarge stream and the temperature of the eddy current torque couplingdischarge stream.
 10. The system of claim 1, wherein the supercriticalfluid is carbon dioxide.